Post polymerization cure shape memory polymers

ABSTRACT

This invention relates to chemical polymer compositions, methods of synthesis, and fabrication methods for devices regarding polymers capable of displaying shape memory behavior (SMPs) and which can first be polymerized to a linear or branched polymeric structure, having thermoplastic properties, subsequently processed into a device through processes typical of polymer melts, solutions, and dispersions and then crossed linked to a shape memory thermoset polymer retaining the processed shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/387,256, filed Dec. 21, 2016, which is a continuation application ofU.S. application Ser. No. 13/892,719, filed May 13, 2013, now U.S. Pat.No. 9,540,481, issued Jan. 10, 2017, which is a divisional of U.S.application Ser. No. 13/099,146, filed May 2, 2011, now U.S. Pat. No.8,883,871, issued Nov. 11, 2014, which claims priority to U.S.Provisional Application No. 61/332,039, filed May 6, 2010, entitled“Shape Memory Polymers That Cure Post Polymerization”. The entirecontents of each of the above applications is hereby incorporated byreference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory and supported by the NationalInstitutes of Health/National Institute of Biomedical Imaging andBioengineering Grant R01EB000462.

BACKGROUND Field of Endeavor

This invention relates to chemical polymer compositions, methods ofsynthesis, and fabrication methods for devices regarding polymerscapable of displaying shape memory behavior and which can first bepolymerized to a linear or branched polymeric structure, havingthermoplastic properties, subsequently processed into a device throughprocesses typical of polymer melts, solutions, and dispersions and thencrosslinked to a shape memory thermoset polymer retaining the processedshape.

State of Technology

Shape memory polymers (SMPs) are useful for a diverse set of engineeringapplications. Because SMPs can retain fixed secondary shapes and recovertheir original shapes upon heating, their applications are oftendirected at, but are not limited to, the biomedical industry. Forexample, an SMP-based suture anchor for graft fixation called Morphix®received FDA approval in February 2009 and has recently been implantedinto humans for the first time.⁹ An SMP-based interventionalmicroactuator device for treating ischemic stroke³ is currently beingsubjected to animal testing at the Texas A&M Institute for PreclinicalStudies. SMPs have also received attention for applications outside themedical industry. Raytheon© is currently investigating SMP foams forimplementation in thermally-activated wing-deployment systems.

While much progress has been made in the development of new shape memorypolymers (SMPs) for engineering applications, difficulties in SMPprocessing have occurred because many chemically crosslinked SMPs arecurrently produced in a one-step polymerization of monomers andcrosslinking agents. Covalently bonded chemically crosslinked SMPs offernumerous advantages over physically crosslinked SMPs, which includesuperior cyclic recoverable strains, higher rubbery modulus values, andhigher toughness values. These thermoset SMPs are traditionallysynthesized either by photo-polymerization or heat-curing of liquidmonomers. The chemical reactions that occur during polymerization oftenresult in volume change, which makes complex molding difficult.Thermoset polymers cannot be melted down, so traditional thermoplasticprocessing methods such as injection molding cannot be used to re-shapechemically crosslinked SMPs to fix deformities. Ultimately, currentproblems in SMP synthesis have limited the mass-production of complexSMP devices. Without the use of injection molding, the mass-productionof complex SMP-based products is neither economically feasible noradvantageous.

What is needed, therefore, is a material that can be melt-processed as athermoplastic and then crosslinked during a secondary step to fix itsfinal shape. This idea of inducing chemical crosslinking intothermoplastic polymer chains is not in itself novel: it dates back tothe 19th century, when the process of vulcanization was developed byCharles Goodyear.¹⁶ Late 20th Century projects such as those of Le Roy(Le Roy, et al, Societe Nationale des Poudres et Explosifs (Paris, FR),United States, 1982) and Goyert (Goyert et al, Bayer Aktiengesellschaft,Levertusen DE, United States, 1988) achieved successful crosslinking ofthermoplastic polyurethanes and acrylates using irradiation, andBezuidenhout, et al. U.S. Pat. No. 7,538,163 in 2009 for the developmentof other chemical mechanisms of post-polymerization urethanecrosslinking.

Other have, more recently, investigated post-polymerization crosslinkingin thermoplastic polyacrylate systems. However, none of these works havespecifically aimed to apply the concept of post-polymerizationcrosslinking to the synthesis, characterization, and optimization of thethermo-mechanical properties of shape memory polymers with transitiontemperatures in the range relevant for biomedical applications bytailoring the chemistries of the polymer systems to maximizesusceptibility for post-polymerization crosslinking. Furthermore, to ourknowledge no prior work had the objective to place crosslinking sitespredominantly uniformly spaced along the polymer chain to provide verysharp actuation transitions.

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The object of this invention are chemical compositions, methods ofsynthesis, and fabrication methods for devices regarding polymerscapable of displaying shape memory behavior and which can first bepolymerized to a linear or branched polymeric structure, havingthermoplastic properties, subsequently processed into a device throughprocesses typical of polymer melts, solutions, and dispersions and thencrosslinked to a shape memory thermoset polymer retaining the processedshape. Suitable processes include solution casting, dip coating,thermoforming, compression molding, injection molding, extrusion, andfilm blowing.

After having been processed into a particular article or shape, theseSMPs are able to be crosslinked or cured so that this shape becomes thepermanent shape of the article having thermoset properties. Setting ofthe shape (curing) is possible through a number of processes includingbut not limited to photo-curing, heat based curing, phase separation(for multiphase/segmented systems), and the like. Specific curechemistries include but are not limited to: thermally or radiativelyinitiated radical cure of vinyl groups utilizing a radical initiator,peroxide or sulfur based crosslinking of vinyl groups, thiol addition tovinyl, reaction of isocyanate containing curing agents with hydroxyl,carboxylic acid, amine, or other functionality on the polymer chains;condensation of ester linkages, epoxy chemistry, silane and siloxanecoupling reactions, Diels-Alder-type cyclizations, mechanically-inducedchemical reactions such as ultrasound-induced electrocyclicring-openings, the encapsulation of any crosslinking agent insidemicrocapsules dispersed in the thermoplastic for subsequent activatedcure, and the like.

In broad aspect the invention, in one embodiment, is a thermoplasticlinear or branched linear polymer having shape memory properties andhaving crosslinkable sites substantially regularly spaced along thepolymer chain which when crosslinked by suitable cure or crosslinkingmeans forms a thermoset polymer having shape memory properties.

In another it is a method of making polymeric articles having shapememory properties comprising forming a thermoplastic linear or branchedlinear polymer having shape memory properties, processing the polymerinto a desired shape, curing the polymer so that a thermoset polymer isformed that has the desired shape as the permanent shape and may be madeto take a stable secondary shape through the application of stress orstrain at a temperature above its actuation transition, then held at thesecondary shape while cooled to a temperature below its transition.

The advantages of the ability to form stable shapes with these uniquepolymers lends them a multitude of uses including interventional medicaldevices, consumer goods, toys, insulation, expandable structures foraerospace applications, actuating devices such as components ofmicrodevices, used for bioanalytical instrumentation and sensors. Thesepolymers also have the potential for use in shape memory polymer foamswith improved toughness.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 graphical representation showing comparison of synthesis andprocessing of a traditional shape memory polymer and a post-condensationcured shape memory polymer of this invention.

FIG. 2 is a plot showing gel fraction versus % DCHMDI for samples of anembodiment of the invention.

FIG. 3 is a plot showing DSC results for Series 1 thermally crosslinkedsamples.

FIG. 4 is a plot showing Storage modulus for thermoplastic, radiationcrosslinked, and thermally crosslinked in compositions of thisinvention.

FIG. 5 is a plot of DMA storage modulus (G′) for thermally crosslinkedsamples of the invention.

FIG. 6 is Tan delta plots for thermally crosslinked samples ofcompositions of this invention.

FIG. 7 is a plot showing the effect of increasing DCHMDI composition onradiation crosslinking of select compositions of this invention.

FIGS. 8 a and 8 b are plots showing percent recovered strain versustemperature for two compositions of this invention.

FIG. 9 is a plot showing recovery stress versus temperature forthermoplastic and radiation for a composition of this invention.

FIG. 10 is a picture of images of the shape recovery at 37° C. of acomposition of this invention over a 12-second time period.

FIG. 11 is a graphical representation of a proposed chemical mechanismfor the radiation crosslinking of samples containing 2-butene-1,4-diol.

FIG. 12 is a plot of showing the effect of increased radiationsensitizer composition storage modulus for samples irradiated at 100kGy.

FIG. 13 is a plot showing the effect of increased radiation dose onstorage modulus for thermoplastic samples solution blended with 5% TAcIC

FIG. 14 is a plot showing the effect of increased PETA composition onstorage modulus for thermoplastic samples irradiated at 50 kGy and witha low molecular weight (M_(w)=7009).

FIG. 15 is a plot showing the effect of the presence of a double bond inthe beta position to the carbamate group in the diol segment of thepolyurethane on storage modulus for thermoplastic samples irradiated at100 kGy and solution blended with 20% sensitizer.

FIGS. 16 a and 16 b are plots respectively showing independence ofrubbery modulus (a) and glass transition temperature (b).

FIG. 17 is a picture showing a Complex medical device made from moldinga a composition of this invention and then irradiating it at 50 kGy.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

This invention, in broad aspect, is both shape memory thermoplasticpolymer compositions that can be shaped then cured into a permanentthermoset shape memory polymer and the method of making such polymercompositions.

A comparison of traditional chemically cured (crosslinked) SMPs and thenovel SMP compositions made according to this invention is shown in FIG.1 . Both thermally activated and radiation-induced crosslinking methodsare disclosed. As used herein the terms cured and crosslinked are usedinterchangeably.

The compositions of the invention are achieved by producing athermoplastic linear or branched linear polymer having shape memoryproperties and having curable (crosslinkable) sites substantiallyregularly spaced along the polymer chain which when crosslinked bysuitable cure or crosslinking means forms a thermoset polymer alsohaving shape memory properties.

This invention provides chemical compositions, methods of synthesis, andfabrication methods for devices regarding polymers capable of displayingshape memory behavior and which can first be polymerized to a linear orbranched polymeric structure and then subsequently processed into adesired shape through processes typical of polymer melts, solutions, anddispersions. Such processes include but are not limited to solutioncasting, dip coating, thermoforming, compression molding, injectionmolding, extrusion, and film blowing.

After having been processed into a particular article or shape, theseSMPs are able to be crosslinked or cured so that this shape becomes thepermanent shape of the article. Setting of the shape is possible througha number of processes including but not limited to photo-curing, heatbased curing and phase separation (for multiphase/segmented systems).Specific cure chemistries include but are not limited to: thermally orradiatively initiated radical cure of vinyl groups utilizing a radicalinitiator, peroxide or sulfur based crosslinking of vinyl groups, thioladdition to vinyl, reaction of isocyanate containing curing agents withhydroxyl, carboxylic acid, amine, or other functionality on the polymerchains; condensation of ester linkages, epoxy chemistry and silane andsiloxane coupling reactions, Diels-Alder-type cyclizations,mechanically-induced chemical reactions such as ultrasound-inducedelectrocyclic ring-openings, the encapsulation of any crosslinking agentinside microcapsules dispersed in the thermoplastic for subsequentactivated cure, and the like.

The ability of these materials to be crosslinked after initialpolymerization may be due to the presence of a second type of functionalgroup on the original monomer, or it may be due to inherent residualreactivity in the system that can be utilized through the application ofenergy such as radiation.

A key aspect of these materials is that they are initially formed intorelatively high molecular weight chains prior to curing, providing forfabrication with typical polymer melt or solution processing methods.This also allows for simultaneously recovery of very high strains aswell as a very high percent recovery of strain.

Another key aspect of these new crosslinkable SMPs is an improvement inboth the extensibility and toughness of the material versus thosethermoset SMPs formed directly from monomers, dimers, and other lowmolecular weight precursors.

Yet, another key aspect of these new crosslinkable SMPs is that theplacement of crosslink sites along the chain can be very regular (e.g.constant Mw between crosslinks), providing for very sharp thermaltransitions for actuation.

There are a multitude of uses for the materials described in thisinvention including interventional medical devices, consumer goods,toys, insulation, expandable structures for aerospace applications,actuating devices such as components of microdevices, used forbioanalytical instrumentation and sensors. These polymers have thepotential for use in shape memory polymer foams with improved toughness.

The compositions and methods of embodiments of this invention areprincipally explained in this disclosure by polyurethane compositionsbut the invention includes, as well, polymer systems includingcondensation polymer compositions that include polyesters, polyamides(e.g. Nylons), polyaramides, polyureas, polycarbonates, polyethers,epoxies and vinyl polymer systems including homopolymers (e.g.hydroxyethylmethacrylate) and copolymers (e.g. alternating copolymers).

One set of polymer compositions of this invention will be those whereinthe polymer backbone or polymer side chain contains, in the followingorder: an electron withdrawing group, a methylene or methyne carbon inthe alpha position to the electron withdrawing group, and an unsaturatedcarbon-carbon double bond in the beta position to the electronwithdrawing group. For radiation-induced curing, the double bondprovides resonance stabilization for radiation-induced radicals formedfrom hydrogen extrapolation at the alpha methylene or methyne carbon,enhances the polymer's susceptibility to crosslinking viaradiation-induced radical graft polymerization, and allows crosslinksites to be incorporated into the polymer chain at uniform intervals.

Another set of suitable polymer compositions of this invention will bethose wherein the polymer backbone or polymer side chain contains anunsaturated carbon-carbon double bond. This double bond should bethermodynamically stable enough to remain unreactive during the initialthermoplastic polymerization. For thermal cure, the double bonds act asa crosslinking site for crosslinking via thermally-activated radicalchain polymerization.

In another set of suitable polymer composition the crosslinkable sitesare alkoxysilanes or acetoxysilane based. The alkoxy- or acetoxy-silanegroups can be incorporated into the SMP backbone in multiple ways.First, they can be incorporated using a diisocyanato-dialkoxysilane,where the isocyanate groups react with the normal diol and become partof the chain. The remaining one or two alkoxy (or acetoxy) groups on thesilicone atom then are available for a moisture cure mechanism. In themoisture cure, the alkoxysilane/acetoxysilane groups react with water toform silanols, then two silanols from separate chains condense to form acrosslink and kick off water.

A fourth means is to have a tri (alkoxy or acetoxy) hydrogensilanemolecule react via Pt catalyzed addition with vinyl groups already onthe linear polymer to create mono-, di-, or tri-(alkoxy or acetoxy)silane side groups. This would then also be moisture curable accordingto the reactions above.

Polyurethane Systems

Specific monomers which can be used for urethanes withpost-polymerization olefinic (carbon-carbon double bond) basedcrosslinking include 1,6-diisocyanatohexane (HDI),trimethylhexamethylene diisocyanate, dicyclohexylmethane 4,4′diisocyanate, trans-1,4-cyclohexylene diisocyanate,1,3-Bis(isocyanatomethyl)cyclohexane, 1,5-Diisocyanato-2-methylpentane,1,7-diisocyanatoheptane, 1,8-Diisocyanatooctane, 2-butene-1,4-diol,1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol,1,6-hexanediylbis[oxy(2-hydroxy-3,1-propanediyl)] bisacrylate, bisphenolA glycerolate dimethacrylate, 3,4-Dihydroxy-1-butene, 7-Octene-1,2-diol,pentaerythritol triacrylate, diethylene glycol, diethanolamine,hydroquinone bis(2-hydroxyethyl) ether, triethylene glycol,1-(benzyloxymethyl) triethylene glycol and 2,2′-ethyliminodiethanol. Toincrease radiation-induced crosslink density, the thermoplastics can besolution blended in THF or other solvents with polyfunctional (meth)acrylate sensitizers such as pentaerythritol triacrylate,tris[2-(acryloyloxy)ethyl] isocyanurate, triallyl isocyanurate,diurethane dimethacrylate, 1,6-hexanediol diacrylate, or othercustom-synthesized sensitizers, but radiation crosslinking can still beachieved without the use of sensitizers.

EXAMPLE 1

Exemplary of compositions of this invention, linear, olefinic urethanepolymers from 2-butene-1,4-diol, other saturated diols, and variousaliphatic diisocyanates including trimethylhexamethylene diisocyanate(TMHDI), and dicyclohexylmethane 4,4′-diisocyanate (DCHMDI) weresynthesized. The chemical structures of these monomers are illustratedin Table 1. Monomers were selected which were predicted to producepolymers with glass transitions in the range of 20-80° C. Urethanechemistry was selected because of the high relative thermodynamicstability of the vinyl group in 2-butene-1, 4-diol relative to thestability of the isocyanate/diol reaction and in order to incorporatecrosslink sites along the chains at substantiality uniform intervals.This unsaturated (double bond) site was expected to remain unreactiveduring the initial polymerization and thus be preserved in the polymerbackbone. These compositions were then cured by crosslinking at or nearthe unsaturated sites as described below.

TABLE 1 Compositions of Series 1, 1R, 1H, 2, 2R, 3, and 3R samples.Radia- Series Un- Heat tion 1, 1H, cross- Cross- Cross- 1R DCHMDI TMHDIlinked linked linked 50% 2-  0% 50% 1a 1H-a 1R-a butene-  5% 45% 1b 1H-b1R-b 1,4- 10% 40% 1c 1H-c 1R-c diol 20% 30% 1d 1H-d 1R-d 30% 20% 1e 1H-e1R-e 50% 1,4  0% 50% 1f — 1R-f butone- diol 1,8- 2- Series octane-butene- 2, 2R diol 1,4-diol 50%  5% 45% 2a — 2R-a TMDHI 15% 35% 2b —2R-b 20% 30% 2c — 2R-c 25% 25% 2d — 2R-d 1,6- 2- Series hexane- butene-3, 3R diol 1,4-diol 50% 10% 45% 3a — 3R-a TMHDI 15% 35% 3b — 3R-b 20%30% 3c — 3R-c 25% 25% 3d — 3R-d Chemical Structures Diols

Diisocyanates

These thermoplastics were melt-processed into desired geometries andthermally crosslinked at 200-225° C. or radiation crosslinked at 50 kGy.The SMPs were characterized by sol/gel analysis, differential scanningcalorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing,and qualitative shape-recovery analysis. Sol/gel analysis and DMAresults provided concrete evidence of chemical crosslinking, and furthercharacterization revealed that the urethanes had outstanding mechanicalproperties. Key properties include tailorable glass transitions between25 and 80° C., tailorable rubbery moduli between 0.2 and 4.1 MPa,recoverable strains approaching 100%, failure strains of over 500% atT_(g), and qualitative shape-recovery times of less than 12 seconds atbody temperature (37° C.). Because of its outstanding thermo-mechanicalproperties, one polyurethane was selected for implementation in thedesign of a complex medical device. These new post-polymerizationcrosslinkable urethane SMPs constitute an industrially relevant class ofhighly processable shape memory materials.

For some embodiments, target mechanical properties included a glasstransition temperature (T_(g)) below body temperature (37° C.), a sharpglass transition range, a high rubbery modulus, a high strain to failureat T_(g), a high recoverable strain capacity, a high recovery stress,and a fast shape recovery time at body temperature. Dynamic MechanicalAnalysis (DMA) and solvent extraction experiments were carried out inorder to confirm the occurrence of post-polymerization crosslinking andto characterize this novel crosslinking mechanism. Further DMA tests, aswell as DSC, tensile testing, and qualitative shape-recovery analysisexperiments were run to evaluate the biomedical relevance of the newurethane materials.

EXPERIMENTAL

Materials and Thermoplastic Sample Preparation

Thermoplastic urethane samples were synthesized from monomers whichcould possibly result in post-polymerization crosslinking. Threedistinct series of materials were synthesized. Series 1a-1e was preparedfrom 2-butene-1, 4-diol (95%) and varying ratios of TMHDI, (97%, TCIAmerica), and DCHMDI, (97%, TCI America). Series 1a-1e consisted of 0%,5%, 10%, 20%, and 30% DCHMDI (overall molar percent). Increasing DCHMDIcomposition was predicted to raise the T_(g). Sample 1f was preparedfrom TMHDI and 1,4-butanediol (98%) in order to evaluate the effect ofthe double bond in 2-butene-1,4-diol on crosslinking. Series 2 wasprepared from TMHDI and varying ratios of 2-butene-1,4-diol and1,8-octanediol (98%). Series 2a-2d consisted of 5%, 15%, 20%, and 25%1,8-octanediol (overall molar percent). Series 3 was prepared from TMHDIand varying ratios of 2-butene-1,4-diol and 1,6-hexanediol (98%). Series3a-3d consisted 10%, 15%, 20%, and 25% 1,6-hexanediol (overall molarpercent). The saturated diols were added to lower the T_(g). Thechemical compositions of all samples are listed in Table 1.

All chemicals, unless otherwise stated, were purchased fromSigma-Aldrich and used as received. All urethanes were prepared in 50%solution in tetrahydrofuran (THF; anhydrous, >99.9%) usingstoichiometric diisocyante/diol ratios. The isocyanate monomers werestored under dry nitrogen until use to prevent moisture absorption. Thestoichiometric diol-diisocyanate solutions were prepared in glass vials.The vials were loosely sealed (to prevent pressure buildup) and wereplaced in a Thermoline furnace at 60° C. under a dry nitrogen atmospherefor 24 hours. The polymer solutions were then poured into polypropylenedishes and placed into a Yamato Benchtop Vacuum Drying Oven at 80° C. at1 torr for 48-144 hours.

After drying under vacuum, the thermoplastic samples were mostly solventfree. The samples were then removed from the polypropylene dishes andpressed to a thickness of 1 mm using a Carver hot press at 150° C. for20-30 seconds. The samples were pressed between Teflon-coated stainlesssteel plates using a 1 mm-thick square stainless steel spacer.

Preparation of Thermally and Radiation Crosslinked Samples

After the thermoplastic samples were synthesized, they were subjected toheat or radiation in an attempt to induce chemical crosslinking. Thesamples prepared for thermal crosslinking were put back on theTeflon-coated stainless steel plates and placed in the Yamato vacuumoven at 200° C. at 1 torr until the onset of crosslinking was visible.The onset of crosslinking was marked by the failure of bubbles in thesamples to evaporate out. After the onset of crosslinking, vacuum wasreleased, and the samples were left under nitrogen at 200° C. for 10hours. Heat crosslinking only yielded testable, thin-film samples forSeries 1. The 1 mm-thick films were laser-cut into DMA and dog bonesamples using a Universal Laser Systems CO₂ VeraLaser machine. Theheat-crosslinked Series 1 samples were then labeled 1H-a to 1H-e. It isimportant to note that no thermal initiator was used to induce thermalcrosslinking.

Sample 1a was exposed to different temperatures for varying amounts oftime in order to evaluate the effects of temperature and heat exposuretime on crosslinking. In Series 4, thermoplastic 1a samples (0% DCHMDI)were placed in the oven at 200° C. for 1, 2, 3, 4, 6, 8, 10, and 12hours. Samples were labeled Series 4a, 4b, etc. Another series ofthermally crosslinked 0% DCHMDI samples, Series 5, was made from heatexposure 225° C. for 2.5, 4, 6, and 8 hours and was labeled Series 5a,5a, etc. After being pressed to 1 mm-thick films, all thermoplasticsamples in Series 1-3 were exposed to electron beam radiation at 50 kGy.Irradiated samples were labeled 1R-a, 2R-a, etc. Characterization bySol/Gel Analysis

In order to determine if the heated and irradiated samples werecrosslinked, sol/gel analysis experiments were run to determine gelfraction. Sol/gel analysis experiments were run on all samples in Series1H and 1R, as well as on select samples in Series 2R and 3R. Since thethermoplastic urethanes were synthesized in 50% THF solution andremained in solution after polymerization, THF was chosen as the solventfor the sol/gel analysis experiments. 0.5 g samples were massed, put in50:1 THF mixtures in 40 mL glass vials, and heated at 50° C. on a J-KemScientific Max 2000 reaction block at 150 rpm for 24 hours. The swollensamples were then vacuum-dried at 100° C. at 1 torr for 24 hours, untilno further mass change from solvent evaporation was measurable.

Sol/gel analysis and DMA results showed that several of the new urethanesystems were crosslinked. Mechanical characterization revealed that thematerials had mechanical properties highly suitable for biomedicalapplications. While the 1H thermally crosslinked urethanes all had gelfractions above 90%, the 1R radiation crosslinked urethanes showed asignificant decrease in gel fraction as DCHMDI composition was increasedfrom 0-30%. A plot of chemical composition versus percent gel fractionfor Series 1H and 1R is provided in FIG. 2 . Sol/gel analysis data forall samples is provided in Table 2.

TABLE 2 Sol/gel analysis results for all samples tested Sample Gel Fr.Sample Gel Fr. Sample Gel Fr. 1H_a 91.8% 1R_a 93.2% 2R_b 80.2% 1H_b90.5% 1R_b 68.9% 2R_d 95.8% 1H_c 91.3% 1R_c 66.1% 3R_c 72.2% 1H_d 93.9%1R_d 54.0% 1R_f 78.8% 1H_e 93.3% 1R_e 0.0%

Since the butene-1,4-diol was only 95% pure, and since the urethanesamples may have absorbed moisture from the atmosphere before solventevaporation, the evaporation of water and other impurities may have madethe gel fractions appear even lower than they actually were. Thus, thegel fraction results from the thermally crosslinked urethanes (and anyother gel fractions above 90%) are strong evidence of chemicalcrosslinking.

Characterization by Dynamic Mechanical Analysis

DMA experiments were run on all samples subjected to heating orirradiation using a TA Instruments DMA Q800 Series dynamic mechanicalanalyzer controlled by a PC running Q Series software. Test samples werecut from 1 mm-thick films to 5 mm×12 rectangles.

In order to determine if samples were crosslinked, and also to determinestorage modulus and T_(g), the samples were subjected to DMA isostraintests. In the “DMA Multifrequency-Strain” mode, frequency was set to 1.0Hz, strain was set to 0.1%, preload force was set to 0.01 N, andforcetrack was set to 125%. The temperature range was 0-200° C. with aramp rate of 5° C./min. If sample slippage occurred during the glasstransition, the ramp rate was slowed to 2° C./min over the range ofT=T_(g)±10° C., and the sample was re-run. Plots of storage modulus andtan delta versus temperature were recorded using the QSeries software.T_(g) was determined from the peak of the tan delta curves.

DMA results on all heated and certain irradiated samples are shown inFIGS. 3, 4, 5, 6 and 7 . All samples showed curves characteristic ofamorphous polymers, i.e., a glassy region at low temperatures, a glasstransition at higher temperatures, and a rubbery plateau. FIG. 4compares the DMA curves for thermoplastic, radiation crosslinked, andthermally crosslinked 1a samples. These plots show significant changesin the rubbery modulus values before and after heating and irradiation.While the thermoplastic sample 1a melts around 120° C., the irradiatedand heated samples do not flow at temperatures well above T_(g); thisbehavior indicates that significant crosslinking has occurred.

FIG. 5 , a comparison of storage modulus plots for all thermallycrosslinked samples, shows the polymers to have glass transitions from32 to 80° C. and rubbery moduli from 1.9 to 4.0 MPa. The rubbery modulifor the samples remain constant and even increase slightly withincreasing temperature, thus indicating ideal elastomeric behavior. InFIG. 6 , the tan deltas approach zero both above and below T_(g). Thesefigures show no additional transitions, such as those caused bycrystalline melting. The sharpness of the glass transition, as seen inthe tan delta curves, is evidence of a homogenous network structure.This homogeneity arises from the base polymer's being an alternatingcopolymer and is indicative that there is a narrow dispersion ofmolecular weights between crosslink sites.) When coupled with the highgel fraction data listed in Table 2 and displayed in FIG. 2 , the DMAresults in FIGS. 3, 4, 5, 6 and 7 provide decisive evidence that thesamples in Series 1H are chemically crosslinked.

Cyclic Free Strain Recovery Tests

Cyclic free strain recovery experiments were run in tension to evaluatethe difference in percent recoverable strain between the thermoplasticand crosslinked samples. In the “DMA-Strain Rate” mode, frequency wasset to 1.0 Hz, strain was set to 1.5%, and preload force was set to 0.01N. The samples were heated to 35° C. above T_(g) (tan delta peak),strained to 50%, and were then rapidly quenched to 0° C. at −10° C./minwhile maintaining the 50% strain. Then, for free-strain recovery, theapplied force was set to ON, and the temperature was ramped from 0° C.to 140° C. at 5° C./min. For cyclic testing, the samples were cooledback to T_(g)+35° C. at −10° C./min, strained again to 50%, and theprevious procedures were repeated. Percent strain recovered as afunction of temperature and time was recorded using the QSeriessoftware. For thermoplastic samples, 2-cycle experiments were run, andfor crosslinked samples, 3-cycle experiments were run.

Percent recoverable strain was determined during free recovery overrepeated cycles. FIGS. 8 a and 8 b compares the free strain recovery forthermoplastic and thermally crosslinked 20% DCHMDI samples. After thefirst cycle, the thermally crosslinked sample recovered 95.5% strain.After the second and third cycles, the sample recovered 94.8% and 94.6%strain, respectively. The thermoplastic samples did not demonstrate highpercent recoverable strain. After cycle 1, percent recoverable strainwas 46.1%, and after cycle 2, it was 3.1%. Cyclic free strain recoveryplots are shown for thermally crosslinked and thermoplastic 20% DCHMDIsamples in FIGS. 8(a) and (b), respectively.

Constrained Recovery Tests

In order to determine the maximum recovery stress of the samples in thenew urethane system and evaluate the effect of crosslinking on recoverystress, constrained recovery tests were run on samples 1a and 1R-a.Sample 1R-a was chosen because it had the highest overall rubberymodulus value at T=T_(g) +20° C. In the “DMA-Strain Rate” mode,frequency was set to 1.0 Hz, strain was set to 1.0%, and preload forcewas set to 0.01 N. The samples were heated to 75° C., strained to 50%,and were then rapidly quenched to 0° C. at −10° C./min while maintainingthe 50% strain. Finally, the samples were heated from 0° C. to 150° C.at 5° C./min without removing the applied stress. Recovery stress wasrecorded as a function of temperature.

The radiation crosslinked 0% DCHMDI sample was subjected to constrainedrecovery testing because it had the highest rubbery modulus (4.2 MPa) atT=T_(g)+20° C. of any sample characterized in this work. FIG. 11compares the constrained recovery results for the thermoplastic andradiation crosslinked samples. At body temperature (37° C.), therecovery stress of the crosslinked sample was 0.66 MPa (95 PSI), and itsmaximum recovery stress was 0.83 MPa (121 PSI). The thermoplastic sampledid not exhibit a recovery stress.

Characterization by Tensile Testing

To determine toughness values, ultimate tensile strengths, and failurestrains, strain to failure experiments were carried out on Series 1H.Dog bone samples were cut using a CO₂ laser according to ASTM StandardD-412.

Strain to failure experiments were run three times on each sample using100N load cell in a TA Instruments Insight 2® universal tensile tester.Experiments were run at T_(g), which was determined from the peak of thetan deltas from DMA plots.

Strain to failure showed the new urethanes to have high toughness. Allthree samples strained to over 500% elongation, while still exhibitingsignificant strain hardening. Toughness was calculated to be 50.2 MJ/m³.

Characterization by Qualitative Shape Recovery Analysis

Recovery time was measured using qualitative shape recovery analysis.The qualitative recovery analysis was performed on Samples 1R-a and1H-a, which had sharper glass transition curves than any other materialswith T_(g)'s within 5° C. of body temperature. In these tests, flat4×60×1 mm samples were coiled into helical shapes at 70° C. The deformedsamples were then quenched by immersion in an ice water bath to maintainthe helical shapes. The samples were then placed in 37° C. water, andthe shape recovery was recorded using a high-definition digital videocamera.

The coiled samples both achieved full shape recovery in 12 seconds atbody temperature. Images of Sample 1R-a at different points in its12-second recovery period are provided in FIG. 12 (1H-a was tested, butis not pictured). Each sample was deformed into the coiled shape shownat time 0 in FIG. 10 and put in water at 37° C.

Conclusions from the tests in Example 1.

The DMA plots in FIGS. 4, 5, 6 and 7 , cyclic free strain recoverycomparisons in FIG. 8 , and constrained recovery comparisons in FIG. 9are evidence of both the existence of chemical crosslinking and of itseffects on the mechanical properties of the SMP systems. The fact thatall the materials in these plots had over 90% gel fractions is furtherconfirmation that chemical crosslinking occurred.

From the characterization of the radiation-induced crosslinkingmechanism demonstrated in these examples several conclusions could bedrawn. First, the DCHMDI-containing samples did not appear suitable forradiation crosslinking at room temperature. One explanation for theDCHMDI monomer's inability to undergo radiation crosslinking is that theDCHMDI molecules in the polymer backbone experienced chain scissionduring irradiation, which prevented the formation of a large networkstructure. DCHMDI contains two cyclohexyl groups, which induce highstiffness on the polymer chains and therefore increase T_(g). BecauseDCHMDI-containing samples have glass transitions significantly aboveroom temperature, chain mobility is limited, and the probably thatradical-containing chains will interact via radical graft polymerizationto form crosslinks is decreased. The gel fractions of theDCHMDI-containing samples decreased proportionally with increasingT_(g), as indicated in Table 2.

Second, the 2-butene-1, 4-diol monomer appears to be ideal for radiationcrosslinking. Previous published research has shown that e-beamradiation can cause crosslinking in polyurethanes by ionizing the

-hydrogen adjacent to the carbamate oxygen in the urethane backbone andinitiating a radical-based “graft” polymerization (instead of a radicalchain polymerization), where radicals on different carbons formone-to-one chain-linking covalent bonds. The chemical structure of thethermoplastic urethane (Sample 1a) is provided in FIG. 11 (Structure I),and the

-hydrogens are shown in bold.

What is unique about this urethane is that the

-hydrogens are adjacent to the double bond from the 2-butene-1,4-diolmonomer. Consequently, when the radiation-induced radicals form, theradicals theoretically experience extended resonance stabilization alongparts of the alcohol segment and through the carbamate linkages of thepolymer backbone. We have proposed two possible resonance structures,which are Structures II and III in FIG. 11 . This extended resonancestabilization gives the radicals more time to bond to other radicals andconsequently increases crosslinking. The fact that the 1, 4-butanediolsample, 1f-R, had both a lower rubbery modulus at T=T_(g)+20° C. and alower gel fraction than its unsaturated counterpart indicates that theunsaturated group is involved in the crosslinking mechanism.

EXAMPLE 2

To demonstrate that higher crosslink densities in the radiationcrosslinked polymers, radiation sensitizers were solution blended withthermoplastic polyurethanes before irradiation. Linear, olefinicurethane polymers were made from 2-butene-1,4-diol, diethylene glycol,1,4-butanediol, and trimethylhexamethylene diisocyanate (TMHDI).Radiation sensitizers enhance crosslinking because the vinyl groups aremore sensitive to radiation induced radical formation but also becausethey have a combination of high functionality and small size. Thechemical structures of these monomers are illustrated in Table 2. Afterirradiation, the samples were characterized by the methods described inExample 1.

TABLE 2 Structures for monomers and radiation sensitizers used insynthesis of polymers in Example 2 Monomer Name Structure2-butene-1,4-diol

diethylene glycol

1,4-butanediol

trimethylhexamethylene diisocyanate (TMHDI)

pentaerythritol triacrylate (PETA)

tris[2-(acryloyloxy)ethyl] isocyanurate (TAcIC)

Experimental

Thermoplastic urethane samples were prepared from 2-butene-1,4-diol andtrimethylhexamethylene diisocyanate (TMHDI). These monomers wereselected because our previous work had shown the correspondingthermoplastics to be highly susceptible to radiation crosslinking. Toevaluate the effect of the double bond in the polymer backbone onradiation crosslinking, an analog of this thermoplastic was synthesizedfrom 1,4-butanediol and TMHDI. To lower the T_(g) of the samples,diethylene glycol (DEG) was also used as a substitute for2-butene-1,4-diol. Samples were solution blended in THF with radiationsensitizers (TAcIC and PETA) in 2.5%, 5.0%, 10%, 20% and 25% molarratios. To evaluate the effect of molecular weight on radiationcrosslinking, molecular weight was controlled by adding anhydrousmethanol in 1.0-5.0% mole ratios to the initial monomer mixtures.

All chemicals, unless otherwise stated, were purchased from TCI Americaand used as received. All thermoplastic urethanes were synthesized in a33 vol % solution of anhydrous (>99.9%) THF. Zirconium(IV)2,4-pentanedionate was purchased from Alfa Aesar and used as a catalyst(0.01 wt % of monomers) for the urethane polymerization. This catalystwas chosen because it has been shown to favor urethane formation overurea formation when moisture is present. All solvents, alcohol andisocyanate monomers, and catalysts were stored, massed, and mixed underdry air in a LabConco glove box. 100 g samples (total monomer mass) weremassed in the glove box and put in 225 mL glass jars, after which theTHF and Zr catalyst were added. The jars were sealed and were thenplaced in a LabConco RapidVap machine at 65° C. for 24h at a vortexsetting of 25 under dry nitrogen. The RapidVap was used to heat and mixthe monomer solutions under an inert, moisture-free atmosphere. After24h, the viscous polymer solutions were poured into 12″×9″ rectangularpolypropylene (PP) dishes, which were purchased from McMaster-Carr. ThePP dishes were then placed under vacuum at 65° C. for 72h to removesolvent.

After the solvent was removed, the large thermoplastic films were cutinto strips and put into 20 mL glass vials in masses of 4-5 g. Allmasses were recorded, and radiation sensitizer compositions necessary tomake 2.5-25 mole % samples were calculated based on these masses. Thethermoplastic strips were then re-dissolved in THF (33 vol % solution)using the heat and vortex features of the RapidVap overnight at 50° C.and at a 25 vortex setting. The radiation sensitizer monomers were thenadded in appropriate amounts, and the vials were topped off with THF togive a final (polymer+sensitizer): THF volume ratio of 1:4. 3.3 mL ofeach blended polymer solution was then added evenly to each compartmentof 2″×4″×12 compartment PP boxes, which were purchased fromMcMaster-Carr. These volumetric amounts were calculated to give finalfilms of about 0.30 mm thickness. The PP boxes were then placed undervacuum at ambient temperature for 2 days, after which the temperaturewas increased to 45° C. for an additional 2 days. The resultingamorphous thermoplastic films were then placed in 2″×2″×2 milpolyethylene bags. The samples were irradiated using a 10 MeV electronaccelerator at 50, 100, 150, 200, 250, 300, and 500 kGy.

Characterization by Dynamic Mechanical Analysis

Dynamic mechanical analysis experiments were performed on the thin filmsamples using the experimental parameters described in Example 1. AsFIG. 12 shows, both rubbery modulus and T_(g) increased with increasingsensitizer composition. Rubbery moduli as high as 70 MPa wereachievable. As FIG. 13 shows, both rubbery modulus and T_(g) increasedwith increasing radiation dose. As FIG. 14 shows, crosslinking wasachievable for samples with extremely low molecular weights.Consequently, injection molding of these materials should be extremelyeasy. As FIG. 15 shows, the presence of a double bond in the betaposition to the carbamate group in the diol segment of the urethanebackbone resulted in a significantly higher rubbery modulus than thepolymer containing the saturated analog; this plot serves as strongevidence for the validity of the resonance stabilization theorydescribed in FIG. 11 . As FIG. 16 shows, independent control of rubberymodulus and glass transition was achievable; consequently, this newurethane SMP system can be considered a true “SMP system,” as has beendefined in the literature.

Articles made from the polymers systems of the Invention

In another embodiment the invention is shape memory article and devicesmade from the polymer composition of the invention. The polymer allowsshaped articles to be formed and/or processed with the composition inthe thermoplastic state, which is more efficient and less liable to formmodification during process, then cured to a permanent shape memorythermoset state. This ability can be especially important in smallarticle such as medical devices.

In another embodiment the polymer composition of the is invention arefabricated into a porous structure or foam by one or a combination ofprocesses from the group of freeze drying, high inverse phase emulsionfoaming, physical blowing, pore templating utilizing a solid or liquidpore former, solution spinning, stereolithographic patterning,micro-extrusion or ink pen printing, 3 D microdot based printing, orlaser machining.

EXAMPLE3

An experiment was run to demonstrate that a polyurethane composition ofthe invention could be processed as a thermoplastic and thensubsequently crosslinked. Sample 1A was molded into the geometry of acomplex medical device, pictured in FIG. 12 . This device, an artificialoropharyngeal airway device, was exposed to radiation, during which itunderwent radiation-induced chemical crosslinking, and after which itwas shown to exhibit shape memory properties. Qualitative shape-recoveryexperiments were again run on the actual SMP-based airway device, andfull recovery occurred in 14 seconds at body temperature.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes can be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the appended claims. The specification is, accordingly, to beregarded in an illustrative rather than a restrictive sense. Therefore,the scope of the invention should be limited only by the appendedclaims.

The invention claimed is:
 1. A polymer composition comprising: a thiolcrosslinker; and a polymer having carbon-carbon double-bondcrosslinkable sites substantially regularly spaced along a polymer chainof the polymer; wherein the polymer, when crosslinked, forms a thermosetpolymer having: (a) shape memory properties, and (b) a glass transitionrange between 25 and 80 degrees Celsius.
 2. The polymer composition ofclaim 1, wherein: the polymer is a thermoplastic polymer; the polymer isconfigured so crosslinking the carbon-carbon double-bond crosslinkablesites with the thiol crosslinker via a thiol-ene reaction forms thethermoset polymer.
 3. The polymer composition of claim 2, wherein thethermoplastic polymer is a polyurethane.
 4. The polymer composition ofclaim 3, wherein the thermoset polymer is a polyurethane.
 5. The polymercomposition of claim 4 wherein the thermoset polymer is apoly(thioether-co-urethane).
 6. The polymer composition of claim 4wherein the thermoset polymer is a poly(thioether-co-ester).
 7. Thepolymer composition of claim 4 comprising an initiator including atleast one of a UV photoinitiator, a thermal initiator, or combinationsthereof, wherein the initiator is configured to initiate the thiol-enereaction that forms the thermoset polymer.
 8. The polymer composition ofclaim 4, wherein the thermoplastic polymer comprises a reaction productof at least one first monomer and at least one second monomer, wherein:the at least one first monomer includes at least one of hexamethylenediisocyanate (HDI); trimethylhexamethylene diisocyanate (TMHDI);dicyclohexylmethane 4,4′-diisocyanate (DCHMDI); trans-1,4-cyclohexylenediisocyanate; 1,5-diisocyanato-2-methylpentane; or combinations thereof;and the at least one second monomer includes at least one of2-butene-1,4-dio1;1,4-butanediol;1,6-hexanediol;1,8-octanediol;1,10-decanediol;1,6-hexanediylbis[oxy(2-hydroxy-3,1-propanediyl)] bisacrylate, bisphenolA glycerolate dimethacrylate; 3,4-dihydroxy-1-butene; 7-Octene-1,2-diol;pentaerythritol triacrylate; diethylene glycol; diethanolamine;hydroquinone bis(2-hydroxyethyl) ether; triethylene glycol,1-(benzyloxymethyl) triethylene glycol; 2,2′-ethyliminodiethanol, orcombinations thereof.
 9. The polymer composition of claim 8, wherein thethermoplastic polymer is a porous structure.
 10. The polymer compositionof claim 9, wherein the porous structure is a foam.
 11. The polymercomposition of claim 8, wherein the thermoplastic polymer is fabricatedinto a porous structure by at least one of freeze drying, high inversephase emulsion foaming, physical blowing, pore templating, solutionspinning, stereolithographic patterning, extrusion, ink pen printing, 3Dmicrodot-based printing, laser machining, injection molding, orcombinations thereof.
 12. The polymer composition of claim 4, whereinthe thermoset polymer has a glass transition range between 32 and 42degrees Celsius.
 13. The polymer composition of claim 1, wherein thepolymer is a porous thermoplastic polymer.
 14. The polymer compositionof claim 13, wherein the porous thermoplastic polymer is a foam.
 15. Thepolymer composition of claim 13, wherein the thermoplastic polymer is anextrusion.
 16. The polymer composition of claim 13, wherein thethermoplastic polymer is a print.
 17. The polymer composition of claim13, wherein the thermoplastic polymer is an injection molding.
 18. Thepolymer composition of claim 1, wherein the carbon-carbon double-bondcrosslinkable sites are substantially regularly spaced along a backboneof the polymer chain.